1. Field of the Invention
This invention relates to a chuck, comprising a base unit with an axis. The base unit has an add-on piece for the installation of the base unit on a chuck mount, in particular on a machine tool. On the end of the base unit away from the add-on piece, there is also a jaw convergence sleeve, whereby inside the jaw convergence sleeve there are also clamping jaws, whereby the clamping jaws are also guided by radially outward friction surfaces on the inside of the jaw convergence sleeve on convergence tracks, which enclose an acute angle with the axis. The clamping jaws also have radially inward tool clamping surfaces to exert a radial clamping force on the shank of the tool to be clamped, whereby the clamping jaws also have thrust absorption surfaces on the ends facing the add-on piece. The thrust absorption surfaces of the clamping jaws are also opposite the thrust surfaces of a thrust body, whereby the thrust body can be moved axially by drive means relative to the jaw convergence sleeve, to bring the tool clamping surfaces closer together to exert a clamping force against the shank of the tool in question.
2. Background Information
A chuck of this general type is disclosed in German Laid Open Patent Application No. 30 48 274 A1, and also in various instances which are or were in public use in at least the Federal Republic of Germany.
In a known configuration, the drive means essentially consists of a jackscrew actuator located in the base unit. This jackscrew actuator comprises an internally threaded sleeve being mounted in the base so that the sleeve can rotate but cannot move axially, and is mounted on the base by means of a thrust ball bearing. In the internally threaded sleeve, there is an externally threaded spindle, held by the engagement of the threads. The externally threaded spindle is made in one piece with a thrust body. The thrust body lies with thrust surfaces oriented orthogonally in relation to the axis of the chuck, in a thrust-transmitting connection with the thrust absorption surfaces of the clamping jaws. The thrust surfaces are an integral component of L-shaped grooves which run radially and orthogonally in relation to the axis, while the thrust absorption surfaces of the clamping jaws are integral components of complementary L-shaped profiles on the clamping jaws. The clamping jaws can thus move radially in relation to the thrust bodies, and can be moved by the thrust bodies in both axial directions. The jaw convergence sleeve is bolted to the base unit so that it cannot rotate. In the base unit, oriented tangentially to the internally threaded sleeve, there is a worm shaft which is engaged with a worm gear thread of the internally threaded sleeve. By turning the worm shaft, the internally-threaded sleeve can be made to rotate. The externally-threaded spindle and the thrust body connected to it as a single piece are held by means of the clamping jaws so that they cannot rotate in relation to the jaw convergence sleeve. A rotation of the internally threaded sleeve, initiated by activating the worm shaft, thus causes an axial movement of the externally threaded spindle, the thrust body, and the clamping jaws which are connected to the thrust body, so that each of these components move axially together. The clamping jaws thereby move independently of the direction of the axial movement of the externally threaded spindle, guided by the convergence tracks of the jaw convergence sleeve, so that, as a function of the direction of rotation of the internally threaded sleeve, the clamping jaws are either moved toward the shank of a tool to be clamped, or are raised away from it.
The known embodiment just described, which by and large has proven reliable, nevertheless has several problems.
The first problem is as follows: The clamping jaws are moved by the wedging action between the convergence tracks and the radially outward friction surfaces of the clamping jaws in the radial direction toward the shank of the tool to be chucked, and are pressed against the tool shank. In the phase during which pressure is applied, there tends to be a high friction force between the thrust surfaces and the thrust absorption surfaces of the clamping jaws, the high friction force being caused by the high axial thrust between the two surfaces. This friction force tends to act against the radially inward clamping force on the shank of the tool to be chucked caused by the wedging action between the convergence tracks and the radially outward friction surfaces of the clamping jaws, caused by the wedging action. In other words, the radial clamping force exerted by the clamping jaws on the respective tool shank tends to be lower on the ends of the clamping jaws toward the thrust body than would correspond to a uniform distribution of the wedging force acting on the clamping jaws over the entire length of the jaws. That can often lead to situations in which the tool shank is not clamped as tightly as desired in these terminal areas of the clamping jaws.
An additional problem is as follows: The worm shaft both causes the clamping jaws to be brought closer to the tool shank, and also generates the clamping force with which the clamping jaws are in contact with the tool shank. The clamping force exerted on the tool shank must essentially be very great, in view of the high torques which are transmitted between the chuck and the tool during operation. That means that between the worm shaft and the worm gear of the internally-threaded sleeve, essentially an extremely high transmission ratio must be selected to be able to apply the necessary clamping forces. This extremely high transmission ratio also means that very high speeds of revolution are necessary on the worm shaft to produce a large radial adjustment movement of the clamping jaws, such as if a previously-clamped tool with a large shank diameter is being replaced by a new tool with a significantly smaller shank diameter. Consequently, the chuck tends to become more complicated to operate. Even if the transmission ratio on the worm gear is set very high to be able to apply high clamping forces by means of the clamping jaws, the clamping forces applied to the clamping jaws by turning the worm shaft by means of a lathe tool are not always sufficient. It should be noted that it is frequently necessary to allow the chuck, as an add-on piece of a rotating machine tool spindle, to rotate in different directions. But if the clamping force is produced, among other things, by the threaded engagement between the externally-threaded spindle and the internally-threaded sleeve, the torque which must be transmitted has a tendency to increase the radial clamping force of the jaws only in one direction of rotation.
An additional problem is the following: On the known system just described, of course, the internally-threaded sleeve of the jackscrew actuator is supported in the radial direction by the radial bearing on the base unit. But on the other hand, the externally-threaded spindle, in its radial position, is essentially fixed exclusively by the threaded engagement with the internally threaded sleeve. When determining the play or clearance of the mated threads of the internally-threaded sleeve and the externally-threaded spindle, it is essentially not possible to select a fit which is as close as might be desirable. Particularly, such a close fit would tend to restrict the smooth running of the jackscrew actuator, which smooth running is generally necessary for the operation of the chuck. Consequently, it must generally be expected that the externally-threaded spindle will be capable of certain radial displacements in relation to the axis of the chuck. The radial displacements tend to mean that the axis of rotation of the machine spindle essentially no longer coincides with the axis of the tool clamped in the chuck (positional error=alignment error). The result is noticeable in the form of eccentricities, or radial deviations, on the tool clamped in the chuck.
At very high speeds of rotation of the machine spindle, these radial shifts lead to balance errors, which generally cannot be eliminated by prior balancing of the chuck, since the offset of the externally-threaded spindle in relation to the axis of the chuck tends to assume different values in relation to the radial position and the magnitude of the eccentricity, each time the tools are re-chucked.
An object of the invention is therefore to eliminate the problems which still exist with known chucks.
Under one aspect of the invention, the thrust absorption surfaces of the clamping jaws and the thrust surfaces of the thrust body--considered respectively in a plane containing the axis--preferably enclose an acute angle with the axis, such that when a thrust is exerted by the thrust body on the clamping jaws, causing the clamping jaws to clamp the chuck, a clamping force component directed radially inward is transmitted from the thrust surfaces to the thrust absorption surfaces.
As a result of this measure, when a tool shank is clamped inside the clamping jaws, as a result of the wedging action between the thrust surfaces of the thrust body and the thrust absorption surfaces of the clamping jaws, there is essentially a clamping force component directed radially inward on the clamping jaws. As a result of this clamping force component, the tendency to reduce the clamping force, which occurs as a consequence of the friction forces between the thrust surfaces of the thrust body and the thrust absorption surfaces of the clamping jaws, can essentially be compensated in whole or in part, so that depending on the size of the acute angle enclosed by the thrust surfaces and the thrust absorption surfaces with the axis, the profile of the force curve can essentially be changed in the axial direction of the tool clamping surfaces of the clamping jaws as desired. In addition, on account of the wedge effect between the thrust surfaces of the thrust body on one hand and the thrust absorption surfaces of the clamping jaws on the other hand, the transmission ratio between the point of application of force of the transmission means on the one hand and the clamping jaws on the other hand can be varied, and in particular increased, in the sense of increasing the radial clamping forces exerted on the tool shank by the clamping jaws.
When this description speaks of a tool with a tool shank, it is merely because that is a frequent application of the chuck according to the invention. It is altogether conceivable, however, that in other applications, the clamping jaws will not be used to clamp a tool shank, but perhaps even the shank of a workpiece being worked on by a stationary tool.
There is a particularly favorable distribution of the clamping force on the clamping jaws if, when considered in a plane containing the axis, the thrust absorption surfaces and the thrust surfaces on one hand, and the corresponding convergence track on the other hand, enclose an angle of approximately 90 degrees.
As disclosed in German Laid Open Patent Application No. 30 48 274 A1 indicated above, a thrust absorption surface and a corresponding thrust surface can be portions of the surface of interlocking profiles, which connect the respective clamping jaws so that they are moved axially with the thrust body in both axial directions. As a result of this measure, regardless of the direction of operation of the transmission means, the clamping jaws are always essentially carried along by the thrust body in the axial direction, i.e. in particular when the corresponding tool shank is to be removed from the chuck.
As also known by prior public use, at least in the Federal Republic of Germany, it is possible to guide the clamping jaws inside the jaw convergence sleeve by means of a conical expansion cage, which essentially guarantees a radial retraction movement of the clamping jaws away from the part being clamped, when there is a corresponding axial movement of the thrust body. This measure essentially guarantees that when the clamping jaws are released by the respective transmission means, i.e. in particular by the jackscrew actuator, they lift up automatically, away from the shank of the respective tool or workpiece.
In contrast to the embodiment disclosed in German Laid Open Patent Application No. 30 48 274 A1, in which the thrust body is connected to an externally-threaded spindle, the invention also teaches that: the thrust body is connected to an internally threaded sleeve of a jackscrew actuator; an externally threaded spindle is fixed in the base unit so that it cannot rotate; the jaw convergence sleeve is mounted on the base body so that it can rotate but cannot move axially; and the thrust body, and with it the internally threaded sleeve, is connected by the clamping jaws so that it rotates together with the jaw convergence sleeve. This configuration is particularly favorable, because then the thrust surfaces are automatically applied radially outward, without any special configuration means, against the internally threaded sleeve and the thrust body. The overall result is a simpler and more compact mechanical structure.
The conical expansion cage can preferably be connected to the convergence sleeve, i.e. it can be fixed to it both axially and also in the circumferential direction. On one hand, the advantage of such a configuration is that taking into consideration the requirement for continuous contact between the clamping jaws and the convergence tracks, the convergence sleeve can be easily manufactured as a turned part. This configuration is in contrast to another conceivable embodiment, in which the guide profiles to guide the clamping jaws are located directly on the inside of the jaw convergence sleeve. On the other hand, it is also possible that the conical expansion cage can be radially mounted with a cylindrical sleeve extension on the inside circumferential surface of an axial hole in the base unit, and that the internally threaded sleeve can be mounted and axially guided inside the cylindrical sleeve extension. This latter possibility means that, essentially, both parts of a jackscrew actuator can be radially fixed in the radial direction directly on the base unit, and can thus be installed without any balance error. It is a simple matter to also fix the externally threaded spindle to its end, away from the clamping jaws in the radial direction, projecting beyond the internally threaded sleeve, and well within the base unit.
The problem indicated above of the complexity of moving the clamping jaws close to the tool shank to achieve sufficient clamping forces between the clamping jaws and the tool shank or, stated inversely, the problem of not being able to achieve sufficient clamping force between the clamping jaws and the tool shank with a facilitated movement of the clamping jaws both toward and away from the tool shank, can be easily solved by having the externally threaded spindle acted on, in an axial direction, by a mechanism which increases the clamping force. Thus the adjustment by means of the jackscrew actuator on one hand, and the clamping force application means of the mechanism which increases the clamping force on the other hand, are preferably independent of one another. Consequently, the jackscrew actuator can be configured so that it only takes a few rotations, e.g. of the jaw convergence sleeve, to travel a large radial adjustment distance, and so that on the other hand, it only takes a small movement of the mechanism which increases the clamping force to apply a large clamping force.
The idea of using a jaw convergence mechanism on one hand to move the clamping jaws close to the tool shank, and a mechanism to increase the clamping force on the other hand to generate the clamping force, is essentially independent of the idea discussed above, i.e. locating the thrust surfaces and the thrust absorption services at an acute angle to the axis of the chuck. In other words, the incorporation of a jaw convergence mechanism and of a mechanism to increase the clamping force can essentially be applied successfully, even if, as in known arrangements, the thrust surfaces of the thrust body and the thrust absorption surfaces of the clamping jaws are oriented orthogonally in relation to the axis of the chuck. The configuration of the jaw convergence mechanism and of the mechanism to increase the clamping force is essentially not mandatory in the embodiments discussed below. The mechanism to increase the clamping force in particular can be used in various embodiments. It is possible, for example, to use a wedge mechanism, a worm gear transmission, a toggle mechanism or preferably a cam mechanism as the mechanism to increase the thrust. For the jaw convergence mechanism, various types of mechanisms can also be used, whereby in this case a jackscrew actuator is preferably used, e.g. so that the jaw convergence mechanism is essentially formed by a jackscrew actuator oriented coaxially in relation to the base unit, and on one hand acts on the thrust body, and on the other hand is acted on by the mechanism to increase the clamping force.
In one preferred embodiment, the jackscrew actuator can be adjusted by turning the clamping jaw convergence sleeve in relation to the base unit. This capability, which is common on hand-held power drills, can be used on the chucks of the high-powered machine tools in question here precisely because of the addition of the mechanism which increases the clamping force, and therefore the rotational movement applied to the jaw convergence sleeve does not also have to apply the high clamping forces which are required.
In one preferred embodiment of the invention, the mechanism which increases the clamping force preferably comprises a camshaft, mounted essentially radially in relation to the axis of the base unit and mounted in the base unit, with a cam or cam body. The jaw convergence mechanism is in contact with this cam body, and the jaw convergence mechanism can in turn be a jackscrew actuator. The camshaft can also preferably be designed with intervention surfaces, e.g. Allen screws, for the engagement of an Allen wrench to rotate the camshaft. The cam shaft can preferably run through the base unit radially, or, in other words, diametrically. That significantly facilitates the solution to the problem of achieving the correct balance, because a camshaft which runs diametrically through the base unit can be executed with small balance errors, in particular if the camshaft extends over the entire diameter of the base unit. In that case, it is not difficult to achieve complete balance. In addition, as a result of the rotation of the radially or diametrically mounted camshaft, there is practically no change in the out-of-balance condition, with regard to its eccentricity, which is very small anyway, so that the system can essentially always be in correct balance, regardless of the position of the camshaft. Moreover, it is easily possible, even from the point of view of the eccentricity, to achieve the correct balance for operation by performing the balancing operation when the camshaft is in the clamped position. To facilitate handling, it is possible to have the camshaft held by prestressing means in a non-clamped position. The camshaft is then automatically fixed in the clamped position by the self-locking interaction between the cam body and the part of the jaw convergence mechanism upon which it acts. The angle of rotation of the camshaft can be restricted by stops to facilitate handling and to prevent overtightening.
If both a jaw convergence mechanism and a mechanism to increase the clamping force are present, then, when there is a change in the direction of rotation, the problem of the automatic release of the clamping jaws can be easily solved, because the jaw convergence mechanism is locked by clamping the mechanism to increase the clamping force. In the event that the jaw convergence mechanism is activated by torsion on the jaw convergence sleeve, it is advantageous, with regard to the locking of the jaw convergence mechanism by clamping of the mechanism, to increase the clamping force if an axial safety device on the jaw convergence mechanism acts as a brake on rotation between the jaw convergence sleeve and the base unit, when an axial load is exerted on the jaw convergence sleeve by the clamping jaws, as a result of the action by the mechanism which increases the clamping force on the clamping jaws.
The idea that, if a jackscrew actuator acts as the jaw convergence mechanism and/or as the mechanism to increase the clamping force, the two parts of this jackscrew actuator should preferably be mounted individually and independently of one another in the radial direction on the base unit, can be applied with advantage, independently of the characteristics of the invention discussed above, i.e. in particular independently of the acute-angle orientation of the thrust surfaces and the thrust absorption surfaces, and independent of the functional separation between a jaw convergence mechanism and a mechanism to increase the clamping force, in terms of improving the balance of the chuck. This is also true for a particularly preferred configuration, in which an internally-threaded sleeve of the jackscrew actuator is mounted inside a sleeve extension of a conical expansion cage. The idea that: the thrust body is connected to the internally-threaded sleeve; the externally-threaded spindle does not rotate in relation to the base unit; the jaw convergence sleeve is mounted so that it can rotate but cannot move axially on the base unit; and the internally-threaded sleeve is connected by means of the clamping jaws to the jaw convergence sleeve so that they rotate together; can also essentially be applied independently of the other features of the invention.
The idea of mounting a jaw convergence sleeve on the base unit so that it can rotate but cannot move axially is also essentially known, on the basis of prior public disclosure at least in the Federal Republic of Germany. But the teaching that the jaw convergence sleeve can be mounted with an inside circumferential surface on an outside circumferential surface of the base unit by means of a needle bearing, so that it can rotate but not move radially, is essentially new and essentially independent of the other teachings of the invention. This proposal essentially makes it possible, in a relatively simple manner, to reduce the danger of the occurrence of out-of-balance conditions as a result of an eccentric mounting of the jaw convergence sleeve. At this point, it should be noted that if the jaw convergence sleeve is mounted so that it can rotate on the base unit, to adjust the clamping jaws by direct or indirect rotational action on the jaw convergence sleeve, the fit of the jaw convergence sleeve on the base unit essentially cannot be made as tight as might otherwise be desired, to avoid friction resistance. But if, as suggested here, there is a needle bearing between the jaw convergence sleeve and the base unit, then without the risk of excessive friction resistance, there can essentially be a close fit of the needle bearing rollers between the surfaces in contact with it of the base unit and of the jaw convergence sleeve, and thus a radial offset of the jaw convergence sleeve, with the consequence that imbalances can be completely eliminated. For this purpose, it is essentially not even necessary to have a radial prestress exerted on the needles between the surfaces in contact with them radially inward and radially outward. It is essentially sufficient to use needles which have particularly close tolerances and correspond exactly to the annular gap between the ring surfaces of the jaw convergence sleeve and the base unit in contact with it.
In one embodiment of the invention, a mechanism to increase the clamping force preferably acts on a jaw convergence mechanism. But this sequence of the mechanism to increase the clamping force and the jaw convergence mechanism is essentially not mandatory. It would also be theoretically possible to change the sequence, i.e. to place the jaw convergence mechanism in direct contact with the base unit, and to insert the mechanism to increase the clamping force between the jaw convergence mechanism and the clamping jaws.
In particular, the invention essentially relates to a chuck, comprising a base unit with an axis, whereby this base unit is designed for external mounting on a chuck mounting, in particular of a machine tool. A jaw convergence sleeve is centrally located on the base unit, in which axially movable clamping jaws are also located inside the jaw convergence sleeve. For the axial movement of the clamping jaws in the direction of clamping and releasing, there is also a jackscrew actuator extending along the axis, which moves axially together with the clamping jaws, but which allows an essentially radial movement of the clamping jaws in relation to the jackscrew actuator. The clamping jaws are also guided by means of radially outward friction surfaces on the inside of the jaw convergence sleeve on convergence tracks. The convergence tracks enclose an acute angle with the axis, so that the clamping jaws are moved radially inward for a movement in the clamping direction. Also, inside the jaw convergence sleeve, jaw expansion means are engaged with the clamping jaws, so that the clamping jaws can be moved radially outward for a movement in the release direction. The jackscrew actuator includes an externally threaded spindle which can be axially supported against the base unit, as well as an internally threaded sleeve which transmits the clamping force as well as the releasing force. The internally threaded sleeve is guided on an inside circumferential surface which centers the sleeve in relation to the base unit. The externally threaded spindle is also centered on the base unit independently of the threaded engagement with the internally threaded sleeve.
Such a chuck is disclosed in German Laid Open Patent Application No. 36 10 671. In this embodiment of the prior art, when a small-diameter tool shank is clamped, the end of the internally threaded sleeve facing the clamping jaws projects far into the interior of the convergence sleeve, where it is both essentially unguided and unsupported. That can generally lead to faulty gripping and thus to out-of-balance conditions.